The present invention relates to a road vehicle having an anti-lock braking system (ABS) and/or an automatic slip control (ASR) for which the travelling speed on a gradient can be adjusted to a predetermined desired speed by means of an active, electronically controlled brake application.
Road vehicles having systems that maintain the vehicle at a constant speed as it travels on a gradient by automatically applying the brakes ("active braking") are known. Such vehicles are, as a rule, equipped with an anti-locking brake system (ABS), as well as with an automatic slip control (ASR). The components of the ABS and ASR systems, such as the electronic control units, wheel speed sensors, solenoid regulating valves, etc., are used to effect the above-mentioned speed regulation on a gradient. Using the known devices, the driver is relieved from having to actuate the brake pedal constantly when the vehicle is travelling on a gradient. Even with a changing incline, uniform speed is maintained.
In DE-A-24 19 242, an electro-pneumatic brake force control circuit is disclosed that maintains a constant vehicle speed as the vehicle travels on a gradient through active braking so long as neither the gas pedal nor the brake pedal is actuated. The speed regulation on a gradient is switched off immediately when either the brake pedal or the gas pedal is actuated, or as soon as the superimposed ABS becomes activated.
Equipping a vehicle with a "hill descent control mode" is also known from International Patent Application WO 96/11826 A1. This type of speed regulation on a gradient can be switched on manually by the driver with the activation of a switch. This type of regulation is able to maintain the vehicle at a constant speed on a gradient by means of active regulated braking of the vehicle without the driver having to actuate the brake. This system is particularly suitable for off road vehicles.
A disadvantage in the known systems is that due to the weight shift to the downhill axle which occurs when the vehicle is travelling on a gradient, the wheels of the downhill axle are insufficiently braked and the wheels of the uphill axle are excessively braked when the brake cylinders are subjected to the same braking pressure. This may lead to unnecessary activation of the ABS due to the locking of the relieved wheels of the uphill axle.